fix(dashboard): show empty states for overview charts

[wayyoungboy]

Summary

Fixes #370 by showing explicit empty states for the overview charts that previously rendered blank when there was no memory data.

Changes:

• show a no-data state for Growth Trend when growth_trend has no positive counts
• show a no-data state for Memory Categories when category counts are empty or all zero
• add English and Chinese dashboard copy for both states

Verification

cd dashboard
pnpm run type-check
pnpm run build
cd ..
git diff --check

pnpm run lint still fails on pre-existing dashboard Biome diagnostics unrelated to this change, such as src/components/ui/* type-only imports, src/lib/api.ts explicit any, and existing src/routes/index.tsx array-index keys.

[lightzt99]

LGTM.